In the application of material metering machines, load cells are relied upon to provide basic weight measurements. Typically, the material is loaded into a hopper, or a vessel, which is attached to the load cell. As the material passes out of the hopper at the bottom, new material is fed into the top of the hopper. The load cell monitors the weight of the hopper and passes those measurements to a processor which calculates the flow rates from the vessels. One area of technology where the material metering machines are used is for resinous or plastic materials. Typically, the resinous or plastic material is in the form of a pellet, of which there are approximately 30 to 80 to a gram. The pellets are passed though a weigh vessel one at a time, or at most a few at a time. Because the vessels have a high tare weight, which may be several thousand pounds, the weigh board must be able to monitor very minute changes in weight to insure the proper flow rate.
However, in this environment there are many sources of noise, both electrical and mechanical. For example, electrical noise may be introduced by converting the analog signal from the load cell to a digital signal that is used by a feedback loop to control the flow rate of the material through the vessel. Another source of electrical noise is line noise, which is the noise introduced into the system by the power supply.
Measurements of the resinous pellets flowing through the hoppers must be made with great accuracy. This is due to the fact that the resinous pellets are very light. Plastic blending machines, which use plastic or resinous pellets, require very precise measurements that are needed for low recipe ratios and run rates in the presence of noise and high tare weights.
To meet the strict requirements needed by plastic blending machines, special purpose digital signal processors (DSPs), which contain the signal processing and filtering function on a single chip are required to minimize the amount of noise introduced into the system. The single chip devices currently available offer limited filtering capabilities and are relatively expensive. Presently, the single chip DSPs do not provide filtering capabilities for the 50 Hz and 60 Hz line noise introduced into the system by power supply, which is a significant factor when carrying out the precise weight measurements needed for plastic blending machines.
Several methods are currently used to reduce the noise from material metering machines. One method uses active vibration control, wherein undesired noise or other vibrations are reduced or eliminated by adding an equal and opposite amount of noise or vibration. Active vibration control is only used to eliminate mechanical noise, such as the noise introduced by the moving parts of the machine. Additionally, active vibration control systems can be relatively expensive because the systems require specialized components and a great amount of processing power. Furthermore, active vibration control methods do not reduce the noise added to the system from the line voltage.
Another method to remove unwanted noise from machine metering systems uses adaptive notch filtering, also known as Adaptive Line Enhancement (ALE). Under the ALE method, vibrations are converted to a digital signal at a fixed frequency by an analog-to-digital (A/D) converter. The digital signal is then applied to a decimation filter to reduce the number of samples used in subsequent calculations. The decimation filter also provides some anti-aliasing filtering to smooth the signal. The decimated signal is then applied to an adaptive notch filter to enhance the signal by filtering most of the noise and harmonics. Although the ALE method filters out most of the unwanted noise and enhances the rejection characteristics of the filtration, the ALE method does not filter out the noise generated by the line voltage at the 50 Hz and 60 Hz frequencies, which can introduce errors in the flow rate of plastic blending machines. Furthermore, the ALE method requires additional processing power over conventional methods, which increases the cost of the system.
Thus, there is a need in the art for an inexpensive noise reduction method to reduce the noise, both electronic and mechanical, from material metering machines. There is a further need in the art for a noise reduction system for use with material metering machines to filter out line noise associated with the 50 Hz and 60 Hz frequencies to provide an output signal that has the stability to drive material metering machines that require precise measurements needed to achieve low mixing ratios in the presence of high tare weights.